Oysters can handle some, but not all pollution we dump into the nation's estuaries

View full sizeAdult oysters can filter up to 50 gallons of water a day, pulling the water in through the fringed mantle seen between the two halves of this Mobile Bay oyster's shell. Scientists say oysters can remove a great deal of nitrogen pollution from the water column, but many U.S. estuaries have more pollution than oysters can handle. (Ben Raines/Press-Register)

MOBILE, Alabama -- While oysters are known to be one of nature’s best natural filters, new research suggests there are limits to how much pollution they can clean up.

In fact, some of the nation’s estuaries are so overwhelmed with excess fertilizer that it would take more oysters than the bays can hold in order to purify the water.

Adult oysters are known to filter about 50 gallons of water daily. But existing research had never fully addressed how much pollution was removed from the water filtered by an oyster, versus how much pollution passed through the animal’s body back into the water.

In particular, scientists sought to understand how much nitrogen oysters remove from the water. Nitrogen is one of the key nutrients responsible for the creation of the Gulf of Mexico’s dead zone off Louisiana, and a similar dead zone that builds in Mobile Bay each year. Sewage treatment plants, industries such as the ThyssenKrupp steel mill, and farm fertilizers all contribute nitrogen to waterways. In the water, nitrogen fuels the growth of phytoplankton, which are tiny plants.

After 10 years of study, scientists with Auburn University and the Dauphin Island Sea Lab figured out a way to measure how much nitrogen an oyster can remove each day as it pumps water through its body. The scientists also analyzed the isotopes present in the oyster tissue to determine the portion of nitrogen that came from land-based pollution. The field work for the study was done on Cape Cod, Mass.

"There is a real chance for people to misunderstand what we are talking about with shellfish cleaning up the environment. People think, ‘Oh, the oysters are cleaning up pollution, I don’t want to eat them.’ No, nitrogen is a nutrient, it’s a problem when we get too much in the system," but it’s not a poison like mercury, said Bill Walton, a marine biologist with Auburn University who was one of the study’s co-authors.

The problem with nitrogen, he said, is that it fuels the growth of algae, which then dies and falls to the bottom, where it decays. The decay process consumes oxygen, leaving the water with too little oxygen to support fish and other marine life.

"Putting shellfish out in the system is a lot like putting sheep in an overgrown pasture. The sheep are going to graze down the excess plants," Walton said. "Removing nitrogen sounds like you are filtering a pollutant. You’re not. Essentially, the oysters are grazing down excess plants."

The study suggests that oysters and other shellfish can be part of a strategy for managing nitrogen pollution, but cannot solve pollution problems alone.

"In the water bodies that are the most polluted, there’s just not space available. In the most polluted urban estuaries, you just don’t have enough room for the oysters. You would have to blanket the bottom of the entire estuary in oysters," said Dauphin Island Sea Lab scientist Ruth Carmichael, another coauthor of the study.

In part, the study showed the limits of what oyster reefs can do in a system in terms of cleaning up pollution, Carmichael said.

"You can’t say we’ll never have to sewer anything, we’ll never worry about stormwater. We’ll just throw some oysters in the water and have carte blanche to pollute because the oysters will do the cleanup work. That’s not going to happen. They are not going to be our panacea," Carmichael said.

Oysters, like all life, have minimum requirements to live. In many of the nation’s estuaries, those requirements are not being met.

"In places like the Chesapeake, they are struggling to get a natural population back, to restore a historical population," Carmichael said. "You have to ask why the oysters aren’t growing there in the first place. You can’t just throw a bunch of oysters out there and expect them to solve the problem if they won’t grow there."

The challenge in Mobile Bay, Carmichael said, is understanding how much nitrogen is coming into the system, and what the sources are. Work is under way to address those questions.

Then, she said, scientists can calculate how much of that problem the bay’s oyster reefs can be expected to help clean up. ÂÂ